The New York challenger has pointed the finger at Khan's training partner, Manny Pacquiao, and the Bolton fighter has claimed that the allegation implies guilt by association.

"Amir and Manny work with me and my trainers," Roach said. "I know what goes on in my gym and I've had enough of this rubbish. That's why we've issued a libel suit. Paulie brought it up that Amir might be on something because he trains with us. My guys are clean. It's an insult because the success of my fighters is down to hard work."

Khan, who is normally unruffled by pre-fight hype, said: "Malignaggi kept asking pointedly why my body has changed since I've been over here. These people do not realise the work Manny and me put into it. They disrespect the phenomenal training it takes to do the job I do.

"I take in five to six thousand calories of the right stuff a day. It takes hours and hours of training every day to work that off, so that even between fights I never walk around more than seven pounds over the 10st light-welterweight limit.

"Drugs? These people don't get it. It's not even about muscle. I've got less of that now than when I turned pro. I'm so fit that I'm punching harder than ever. The benefits come in the fights. I know I'm ready for more than 12 rounds. The fights are easier than the training. I've always had the heart to dig deep when necessary but now I use my head as well. In my four fights with Freddie so far, I've come on a ton both physically and mentally."

The conditioner and nutritionist Alix Ariza said: "It's just nonsense. Everything we use is legal. These guys just work incredibly hard. That's where they get their strength and power from."

Malignaggi said: "I made my comments and I stand by my comments. I never said Amir was a cheater – it never came out of my mouth.

"Somebody asked me a question, did it cross your mind, have you thought about it, being as he has the same team [as Pacquiao] around him. I said I'd be lying if it never crossed my mind. But he hasn't given me any right to think that."